In recent years synthetic papers have been developed for use in the printing and labeling industries. Synthetic papers offer significant advantages over natural wood pulp paper including, for example, improved print quality, water resistance, tear resistance, and tensile strength. Such materials typically are comprised of polymeric materials such as polyolefins or polyesters.
In the manufacture of certain microporous polyolefin sheet materials, the polyolefin polymers typically are blended with finely divided, water-insoluble filler materials and organic plasticizers. The blend of materials is extruded through a sheeting die to form a continuous sheet comprised of a polyolefin polymer matrix having finely divided, water-insoluble filler material distributed throughout the matrix. A network of interconnecting pores communicates throughout the microporous sheet material. The organic plasticizer, which facilitates the extrusion process, is extracted from the sheet by contacting the sheet with an extraction fluid composition. Conventional extraction fluid compositions include, for example, halogenated hydrocarbons such as 1,1,2-trichloroethylene, perchlorethylene, 1,2-dichloroethane, 1,1,1-trichlorethane, 1,1,2-trichlorethane and methylene chloride; or alkanes such as hexane, heptane, and toluene.
For many end use applications, it is important to remove some or a significant portion of the organic plasticizer from the microporous sheet material. For example, where the microporous sheet material is to be used as a printable sheet, residual plasticizer could negatively impact print quality. Also, where the microporous sheet material is to be used as a layer in a multi-layer laminate structure, for example, as an identification card, a high residual plasticizer content could negatively impact lamination peel strength. Of course, for some end use applications a higher residual plasticizer content may be advantageous.
Likewise, it is desirable that there is minimal residual extraction fluid composition in the microporous sheet. For example, residual extraction fluid should be minimized or eliminated altogether from the microporous sheet material where the microporous sheet material ultimately is to be used as a labeling or packaging material for food or pharmaceutical products. In some manufacturing processes, use of the abovementioned conventional extraction fluid compositions such as the alkanes has been avoided because these materials are flammable, thus requiring special handling and equipment. Also, some of the conventional halogenated hydrocarbons have been identified as substances of very high concern under human health and environmental regulations, such as the Registration, Evaluation and Authorisation of Chemicals (“REACH”) system recently adopted in the European Union. For example trichloroethylene, a level 2 carcinogen, is likely to be listed as a “carcinogenic, mutagenic, or toxic to reproduction” (“CMR”) substance under the REACH system.
In view of the foregoing, it would be desirable to manufacture microporous sheet material using an extraction fluid composition that is easily removable from the final microporous sheet material, is non-flammable, and is not a substance of concern under human health and environmentally regulations.